1. Field of the Invention
The present invention is directed to an oscillator circuit for generating an output signal which executes a periodic oscillation, the oscillator circuit being of the type connectible to a crystal via terminals.
2. Description of the Prior Art
Many oscillator circuits are known which make use of an electrical energized crystal as the oscillator source. Many of these known circuits include means for compensating for the slight energy losses which occur during the mechanical oscillation of the crystal.
In these known circuits, usually the electrical signal corresponding to the mechanical oscillation of the crystal is tapped and re-supplied to the crystal via an amplifier circuit, which gives the signal a suitable phase shift. This is known as a linear solution to the energy loss problem because the oscillation of the quartz is maintained by supplying energy at a suitable phase relationship to the oscillation of the crystal. Although the necessary energy is thereby supplied to the crystal, a damping of the oscillation occurs due to phase errors and due to the low output impedance of the amplifier. This is compensated by a correspondingly increased energy supply. These known oscillator circuits therefore have a low efficiency, and are less suitable for applications wherein a low power consumption is a critical criterion, as is the case, for example, in implantable medical devices such as heart pacemakers and the like.
A further disadvantage of most known oscillator circuits is that these circuits must include two separate terminal pins for the crystal, if constructed as integrated circuits. Each terminal pin increases the space requirement for the integrated circuit, and it is therefore desireable to have an oscillator circuit wherein only one specific terminal pin is required for connection to the crystal. Such a circuit is described in the article "A One-Pin Crystal Oscillator for VLSI Circuits," Santos et al., IEEE Journal of Solid State Circuits, Vol. SC-19, No. 2, April 1984, pages 228-235. In this known circuit, however, there is the risk that the oscillator circuit will not oscillate with the fundamental frequency of the crystal, but instead with a harmonic, which is undesirable.